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# Magnesium sulfate
## Overview
Magnesium sulfate is an essential divalent cation. It functions as a physiological calcium channel blocker, acts as a cofactor in numerous enzymatic reactions, and stabilizes excitable membranes. Clinical effects vary significantly by route (IV/IM vs. oral) and administration rate.
## Primary Indications
* **Hypomagnesemia:** Replacement for symptomatic or severe deficiency.
* **Eclampsia/Preeclampsia:** Prevention and treatment of seizures.
* **Torsades de Pointes:** Immediate intervention for polymorphic ventricular tachycardia associated with QT prolongation.
* **Asthma:** Adjunct for severe acute asthma exacerbations (unresponsive to standard beta-agonist/steroid therapy).
* **Preterm Labor:** Neuroprotection for the fetus (variable institutional guidelines).
## Adult Dosing
* **Hypomagnesemia:** 1–4 g IV/IM. Severe deficiency may require up to 8 g per 24 hours. Replacement protocol varies by facility; usually infused over 1–4 hours.
* **Torsades de Pointes:** 1–2 g IV push over 5–20 minutes, may repeat once.
* **Eclampsia/Preeclampsia:** Loading dose of 4–6 g IV over 15–20 minutes, followed by 1–2 g/hour maintenance infusion.
* **Severe Asthma:** 1.2–2 g IV bolus over 20 minutes as a single dose.
## Pediatric Dosing
* **Hypomagnesemia:** 25–50 mg/kg/dose IV/IM (max 2 g/dose). May repeat every 4–6 hours as needed.
* **Status Asthmaticus:** 25–75 mg/kg IV (max 2 g) over 20 minutes.
* **Torsades de Pointes:** 25–50 mg/kg IV (max 2 g) over 10–20 minutes.
## Dose Adjustments
* **Renal Impairment:** Reduce dose in patients with renal insufficiency (CrCl <30 mL/min). Risk of hypermagnesemia is significantly increased; serial levels are mandatory.
* **Hepatic Impairment:** No specific adjustment necessary.
## Contraindications
* Myasthenia gravis (may exacerbate neuromuscular blockade).
* Heart block or myocardial damage (relative contraindication).
* Anuria/severe renal failure.
## Adverse Effects
* **Common:** Flushing, sweating, hypotension, transient hypotension.
* **Serious:** Respiratory depression (usually at levels >10 mEq/L), loss of deep tendon reflexes (early sign of toxicity), cardiac arrest, heart block.
## Key Drug Interactions
* **Neuromuscular Blockers:** Potentiates effect; risk of prolonged paralysis.
* **Aminoglycosides:** May increase risk of respiratory depression and neuromuscular blockade.
* **Digoxin:** Potential for arrhythmias during rapid IV administration.
* **Calcium Channel Blockers:** May cause severe hypotension.
## Monitoring
* **Serum Magnesium:** Maintain therapeutic range (typically 1.5–2.5 mg/dL; higher for eclampsia).
* **Clinical:** Check patellar reflexes (absence precedes paralysis), respiratory rate (>12 breaths/min), and urine output (>30 mL/hour).
* **Toxicity Treatment:** **Calcium gluconate** (1 g IV over 5–10 min) is the antidote for magnesium toxicity.
## Clinical Pearls
* **Infusion Rate:** Rapid IV bolus (unless in cardiac arrest) can cause cardiac collapse and hypotension. Always infuse slowly unless directed by acute resuscitation protocols.
* **Oral vs. IV:** Oral magnesium is poorly absorbed and primarily acts as a laxative; do not use oral formulations for systemic replacement.
* **Eclampsia Protocols:** These are highly institutional-dependent; ensure familiarity with local nursing and OB/GYN standing orders.
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**Disclaimer:** This information is for educational purposes only. Drug dosing, indications, and institutional protocols change frequently. Always verify the current prescribing information via official pharmacy references (e.g., Lexicomp, UpToDate) and local institutional guidelines before administration.
